1. Field of the Invention
The present invention relates to a semiconductor laser device, and particularly relates to a monolithic type semiconductor device that has a plurality of emitting points, and a method for manufacturing the same.
2. Description of Related Art
As pick-up light sources for optical disk devices and light sources for optical information processing, optical communication and optical measurement, semiconductor laser devices are used. For example, as a pick-up light source for reproducing and recording with respect to a CD (compact disk) and a MD (mini disk), an infrared laser with a wavelength in the 780 nm band is used, and as a pick-up light source for reproducing and recording with respect to a DVD (digital video disk) with a higher density, an infrared laser with a wavelength in the 650 nm band is used.
In order to be compatible with a CD, a MD and a DVD, both an infrared laser and a red laser are necessary in one drive, so that a drive that is provided with optical integration units for both of the lasers generally is used. Whereas, in order to satisfy the requirements for downsizing and cost reduction of the drive, and optical adjustment and simplification of assembly processes and the like, further simplification of the optical integration unit itself has been sought recently. Thus, a dual-wavelength laser device having a configuration, in which the infrared laser with a wavelength in the 780 nm band and the red laser with a wavelength in the 650 nm band are integrated on the same substrate, has been utilized practically so as to contribute to the significant simplification of the optical integration unit.
Moreover, as a trend of an optical disk market, there are strong demands for the conformity to LS (light scribe) for describing a picture or a character on a label of a media by using an infrared laser for CDs and the conformity to the increase of the speeds of DVDs, so that both the increase of the output and the high reliability of the dual-wavelength lasers are required.
As one of the representative causes for inhibiting the high output driving in an utility level, melting destruction on a facet of a resonator, that is, COD (Catastrophic Optical Damage) deterioration is exemplified. This failure mode is generated because a vicinity of the facet of the resonator of the semiconductor laser serves as a region for absorbing light that is generated inside the laser. On a semiconductor surface of the facet of the resonator, a specific deep level that is generated according to absorption of oxygen and oxidation of the surface, and a deep level caused by a defect that is present on the semiconductor surface are generated, thereby substantially narrowing a band gap on the facet. According to heat caused by nonradiative recombination via this deep level, the band gap is decreased further, thereby generating a positive feedback that causes more optical absorption, so that a temperature of the facet is increased steeply, which results in the deterioration of the facet due to its melting.
In order to suppress the COD level, a method of diffusing impurity by solid state diffusion so as to disorder an active layer of the facet part and increase the band gap generally is used.
Further, methods for increasing the band gap by plasma treatment also are disclosed in JP3(1991)-89585A, JP10(1998)-84161A, JP10(1998)-223978A, JP10(1998)-223979A and JP2004-538652A as described below.
JP3(1991)-89585A suggests a technique for enhancing the COD level in an AlGaInP laser by substituting P of the cleaved facet with N and reforming a vicinity of the surface of the facet into GaInN and AlGaInN so as to increase the band gap. According to this manufacturing method, in order to substitute P with N, P firstly is desorbed at a high temperature ranging from 600° C. to 800° C., and at the same time, a NH3 gas that is activated by ECR plasma is supplied to a surface of a sample so as to bond N with the desorbed site.
JP10(1998)-84161A discloses a technique for introducing an element from a facet of a compound semiconductor laser, by using plasma such as electron cyclotron resonance plasma (ECR plasma), which utilizes an interaction between a magnetic field and microwaves. Thereby, the band gap of the semiconductor layer in the vicinity of the facet can be widened more than a band of the semiconductor layer in another region, and also can function as a window region. Moreover, since the semiconductor layer formed by the above-described technique is an excellent high-resistance layer having a low surface level density, a leak current toward the facet can be suppressed, and an effect of suppressing the COD damage can be reduced due to a decrease of a center of the nonradiative recombination on an interface.
JP10(1998)-223978A and JP10(1998)-223979A disclose nitriding the semiconductor layer of the facet of the resonator by irradiation of N2 plasma. It is described that a gap in the vicinity of the facet is widened thereby, and in the case where the active layer includes a quantum well, the active layer of the facet of the resonator can be disordered so as to widen the gap. Further, it also is described that a resistance of the nitrided region is increased, and thus can suppress an effect of a reactive current of the facet region, thereby realizing the high-output and long-life semiconductor laser.
JP2004-538652A discloses a manufacturing method for forming a nitride layer on an any structure of a GaAs semiconductor laser that is constituted of a material system selected from the group consisting of GaAs, GaAlAs, InGaAsP and InGaAs. Similarly to the above-described patent document, this method aims to remove a contamination on the surface and decrease an interfacial recombination velocity by treating with the plasma including nitrogen.
The above-described disordering by the plasma treatment is effective for increasing a band gap on an output facet. However, the prior arts as described in the above patent documents have a problem of decreasing the COD level during aging in high-output dual wavelength lasers including an AlGaAs infrared laser and an AlGaInP red laser that are formed monolithically on the same substrate.
Firstly, none of JP3(1991)-89585A, JP10(1998)-84161A, JP10(1998)-223978A, JP10(1998)-223979A and JP2004-538652A refers to the disordering technique by window formation by the impurity diffusion. All of them disclose forming the nitride-based semiconductor layer in the output facet region so as to widen the gap by utilizing the plasma including nitrogen elements, thereby allowing it to function as a window structure. In the case of forming a nitride semiconductor such as AlGaN, GaN, AlGaInN and GaInN in the output facet region by applying any of the techniques described in those documents to the above-described high-output dual wavelength lasers (the AlGaAs infrared laser/the AlGaInP red laser), it is possible to increase the initial COD level by the expansion of the band gap on the facet.
However, in the case of forming the nitride semiconductor layer that functions also as the window structure, a lattice mismatch between a semiconductor layer and a nitride semiconductor layer of a gain region becomes large, which causes a large distortion of the facet. It is considered that, if continuing aging in this state, a defect caused by the distortion is generated in a region having a high density of light energy, and the COD level is decreased gradually during the aging as the defect is eroded.
Moreover, plasma doping with high energy is necessary for disordering the active layer by the plasma treatment. Thus, by channeling the impurity, the impurity enters also into the active layer that contributes to the emission, which may results in a decrease of the reliability.
Moreover, since the crystallinity of the facet part is decreased after doping the impurity, heat treatment at a high temperature is necessary, but it is difficult to anneal at the high temperature after forming the reflective film on the facet.
Further, recently, it has been understood that a solid state diffusion region affects a beam divergence angle in the facet window structure that is formed by the solid state diffusion. That is, since the beam divergence angle with a low aspect can be obtained by controlling a width of the solid state diffusion region, unless optical confinement of a laser gain part is strengthen, it has become possible to secure the reliability even at high output.
On the other hand, for forming the facet window structure using the plasma doping, the strengthening of the optical confinement of the laser gain part and recovery annealing at a narrow temperature margin after forming the facet reflective film are necessary, so that it is difficult to obtain sufficient reliability at high output.
Further, also in the facet window structure by the solid state diffusion, a new problem occurs according to the recent increase of the output. That is, in a window formation step for disordering the active layer by diffusing the impurity, it becomes possible to increase the initial COD level by a band gap increasing effect due to the disordering of the active layer and an increase of a resistance of the facet. However, a phenomenon of decreasing the COD level occurs during the aging.
It is considered that this is resulted from a point defect due to cutting of a bond by the diffusion of the impurity, generation of a hole and the like, which occur according to the heating generated due to application of power. Thus, even if the initial COD level can be increased by the window formation or the like, the decrease of the COD level during the aging is accelerated, so that the COD level is degraded largely after applying power. Such a degradation inhibits the increase of the output significantly.